The present invention relates generally to memory cells, and more specifically to glass electrolyte memory cells.
A well known semiconductor component is semiconductor memory, such as a random access memory (RAM). RAM permits repeated read and write operations on memory cells. Typically, RAM devices are volatile, in that stored data is lost once the power source is disconnected or removed. Non-limiting examples of RAM devices include dynamic random access memory (DRAM), synchronized dynamic random access memory (SDRAM) and static random access memory (SRAM). In addition, DRAMs and SDRAMs also typically store data in capacitors which require periodic refreshing to maintain the stored data.
In recent years, the number and density of memory cells in memory devices has been increasing. Accordingly, the size of each cell has been shrinking, which in the case of DRAMs also shortens the cells"" data holding time. Typically, a DRAM memory device relies on cell capacity for data storage and receives a refresh command in a conventional standardized cycle, about once every 100 milliseconds. However, with increasing cell number and density, it is becoming more and more difficult to refresh all memory cells at least once within a refresh period. In addition, refresh operations consume power.
Recently, programmable conductor memory elements have been investigated for suitability as semi-volatile and non-volatile random access memory cells. Kozicki et al. in U.S. Pat. Nos. 5,761,115; 5,896,312; 5,914,893; and 6,084,796, disclose a programmable conductor memory element including an insulating dielectric material formed of a chalcogenide glass disposed between two electrodes. A conductive material, such as silver, is incorporated into the dielectric material. The resistance of such material can be changed between high resistance and low resistance states. The programmable conductor memory is normally in a high resistance state when at rest. A write operation to a low resistance state is performed by applying a voltage potential across the two electrodes. The electrode having the more positive voltage applied thereto functions as an anode, while the electrode held at a lower potential functions as a cathode. The mechanism by which the resistance of the cell is changed is not fully understood. In one theory suggested by Kozicki et al., the conductivity-doped dielectric material undergoes a structural change at a certain applied voltage with the growth of a conductive dendrite or filament between the electrodes, effectively interconnecting the two electrodes and setting the memory element in a low resistance state. The dendrite is thought to grow through the resistance variable material in a path of least resistance.
No matter what the theory, the low resistance state will remain intact for days or weeks after the voltage potentials are removed. Such material can be returned to its high resistance state by applying a reverse voltage potential between the electrodes of at least the same magnitude as used to write the cell to the low resistance state. Again, the highly resistive state is maintained once the voltage potential is removed. This way, such a device can function, for example, as a resistance variable memory element having two resistance states, which can define two logic states.
The dendrite will continue to grow after it shorts the electrodes, provided a potential is applied or reapplied. The dendrite""s growth rate before it shorts the electrodes is at a rate much faster than after it shorts, but growth does continue. A problem with certain apparatuses that are capable of using the programmable cell is that if too great a potential is applied, or if the potential is applied for too long a time period, the dendrite becomes sufficiently large and strong that it is not broken by simply reversing the potential. Once this happens, the cell is essentially static, that is, it cannot be reprogrammed with any degree of precision.
There is therefore a need in the art for a way to avoid programming an already programmed programmable cell of the type described.
In one embodiment, a method of programming a glass electrolyte memory cell includes determining a desired value of the memory cell, reading a currently programmed cell value from the cell, and writing the desired value to the cell only if the currently programmed value is outside a predetermined range.
In another embodiment, a method of programming a glass electrolyte memory cell includes determining a desired value of the cell, and reading an actual value of the cell. The desired value and the actual value are compared, and the desired value is written to the cell only if the desired value and the actual value differ.
In yet another embodiment, a method of writing to a glass electrolyte memory cell includes initiating a write operation with a desired write value to the cell, decoding a cell address to be written, reading an actual value of the cell at the decoded address, and comparing the actual value of the cell with the desired value. The desired value is written to the cell if the actual value is outside a predetermined range, and the write operation is terminated without writing to the cell if the actual value is within the predetermined range.
In still another embodiment, a method of operating a glass electrolyte memory cell includes initiating a write operation with a desired cell state, determining, before writing, whether a current state of the cell is the same as the desired state, and determining whether to write the desired cell state to the cell depending upon the current state of the cell.
In still yet another embodiment, a circuit includes a cell reader to read a current state from a glass electrolyte memory cell and a comparator to compare the current state to a desired state of the memory cell.
Other embodiments are described and claimed.